sh_eth: check TSU registers ioremap() error

One must check the result of ioremap() -- in this case it prevents potential
kernel oops when initializing TSU registers further on...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>